How duo were built?
The Twin Towers were the tallest building in the world. It was located in Manhattan, New York City. The idea for the World Trade Center at New York was first proposed in 1943. Though the construction work has been started for first WTC ( World Trade Center) in 1968 and for second WTC in 1969. The construction was finished for first WTC in 1972 and while second WTC in 1973. When the world trade center Twin Towers were completed, the total cost was $900 million and was inaugurated in 1973.
The towers was design as framed tube structures by using steel core and perimeter columns which was protected from the sprayed-on fire resistant material. Closely perimeter provided more strength to the structure which shared gravity load with the columns of the core. It was created by comparatively lightweight structure that could sway more to the wind compared to traditional structure.
One WTC was 1368 feet (417m) and another was 1362 feet and the number of floor was 110.  
How Twin Towers were collapsed ?
The twin towers in New York City were destroyed by terrorist attacks in September 11, 2001. On that fateful morning of September 11, al-Qaeda terrorists hijacked four different passenger planes. By two of these planes, they struck the twin towers in New York City. One of the aircraft struck in North Tower between 93rd and 99th floors at 8:46 am. After seventeen minutes later, another aircraft struck in south tower between 77th and 85th floors at 9:02 am. Both of the towers collapsed within an hour after aircraft struck. Actually how collapsed it? Let’s discuss about it. 
Each of the aircrafts were carrying thousands of gallons of jet fuel which created enormous fireball when the aircraft struck in the twin towers. After the aircraft struck, the heat became intense by the jet fuel. Jet fuel burns at 800 to 1500 degree Fahrenheit temperature though it was not enough for melt the structural steel. But it lost some structural strength from the intensive heat and steel lost half of its strength.
As a result, the core columns slowly lost their strength and were crushed. The perimeter of the columns and floors became weakened by the intensive heat of the fires. The floors began to bow and they pulled the exterior walls inwards. The upper floors crashed down on the lower floors and thus, the entire buildings were collapsed.  
Was aircraft hit, the main reason for collapsed twin towers ?
There are some people who don’t believe that twin towers were collapsed by aircraft struck only. They think that it were collapsed by controlled demolition or by explosive.
The impact of the aircraft didn’t make the towers collapsed entirely. In twin towers building, there were some redundant design, which means that if one system fails, then another system can handle it. So aircraft was not enough for the collapse but the intensive heat forced the structure to lose their strength and as a result, the buildings collapsed.
FEMA suggested that aircraft impact were the key to collapse of the towers. Aircraft were carrying lots of jet fuel which created intensive heat inside the building when it were struck. Though, the heat produced by the burning of jet fuel was not enough for the collapsing structural building. But heat damaged the structural frame and collapsed twin towers.
Marvin Bush and Securacom
"Marvin P. Bush, the president’s younger brother, was a principal in a company called Securacom that provided security for the World Trade Center, United Airlines, and Dulles International Airport. The company, Burns noted, was backed by KuwAm, a Kuwaiti-American investment firm on whose board Marvin Burns also served."
WTC Building 7
An unanswered question is how the building beside the twin towers collapsed. AE911truth.org has compiled the best evidence list with scientific analysis.They concluded that the building had explosions occurring immediately prior to the collapse. Some theories for the reason of this collapse are that there was a planning office for the entire operation by world central banks and international cartel criminals. “In 1999, NYC Mayor Rudy Giuliani constructed a $13 million emergency command retreat on the 23rd floor of World Trade Center Building 7—an armored, self-contained facility designed to provide a safe haven for leadership in the event of a natural disaster or terrorist attack.”